Well drilling operations are typically performed using a long assembly of threadably connected pipe sections called a drillstring. Often, the drillstring is rotated at the surface by equipment on the rig thereby rotating a drill bit attached to a distal end of the drillstring downhole. Weight, usually by adding heavy collars behind the drill bit, is added to urge the drill bit deeper as the drillstring and bit are rotated. Because subterranean drilling generates a lot of heat and cuttings as the formation below is pulverized, drilling fluid, or mud, is pumped down to the bit from the surface.
Typically, drill pipe sections are hollow and threadably engage each other so that the bores of adjacent pipe sections are hydraulically isolated from the “annulus” formed between the outer diameter of the drillstring and the inner diameter of the wellbore (either cased or as drilled). Drilling mud is then typically delivered to the drill bit through the bore of the drillstring where it is allowed to lubricate the drill bit through ports and return with any drilling cuttings through the annulus.
Measurements of formation density, porosity, and permeability frequently need to be taken before a well is drilled deeper or before a change in drilling direction is made. Often, measurements relating to directional surveying are needed to ensure the wellbore is being drilled according to plan. Preferably, these measurements and operations can be performed with a measurement while drilling assembly (MWD), whereby the measurements are made in real-time at or proximate to the drill bit and subsequently transmitted to operators at the surface through mud-pulse or electromagnetic-wave telemetry. While MWD operations are possible much of the time, manual measurements are often desired either for verification purposes, or the measurements desired are not within the capabilities of the MWD system currently in the wellbore. Additionally, measurements may be required when a drillstring is not in the wellbore, for instance during workover or production. For this reason, measurements are often required by “wireline” or other devices absent the presence of the drillstring. Various tools, communications conduits, and method are used in the oilfield today to perform measurements or other operations.
For the purposes of this disclosure, the term “tool” is generic and may be applied to any device sent downhole to perform any operation. Particularly, a downhole tool can be used to describe a variety of devices and implements to perform a measurement, service, or task, including, but not limited to, pipe recovery, formation evaluation, directional measurement, and workover. Furthermore, the term communications “conduit,” while frequently thought of by the lay person as a tubular member for housing electrical wires, in oilfield parlance, is used to describe anything capable of transmitting fluid, force, electrical, or light communications from one location (surface) to another (downhole). For this reason, the term conduit, as applied with respect to the present disclosure is to include wireline, slick line, coiled tubing, fiber optic cable, and any present or future equivalents thereof.
Often, while wireline or other communications conduit operations are being performed, other work and operations continue on the rig floor. Invariably, accidents occur and objects are dropped down the wellbore where the wireline operations are occurring. This can be the result of human error (or, in some circumstances, intentional behavior on the part of rig personnel), or can be the result of the failure of other equipment. No matter how undesired objects get dropped down the wellbore, they must be retrieved, as such objects can often damage or render inoperable any drilling, production, or measurement equipment located downhole. To retrieve these objects, an expensive and time consuming “fishing” operation is undertaken. Fishing involves the deployment of specialty equipment and personnel to “fish” downhole and retrieve the dropped equipment. This process can be simplified if it is known precisely what has been dropped downhole, but this is not always the case.
Also, objects can fall down the wellbore without personnel on the rig even becoming aware of the object downhole until after equipment has been disrupted or damaged. In this circumstance, the expensive and costly fishing expedition is usually followed with an equally expensive and time-consuming retrieval, repair, and replacement procedure to correct the damaged equipment.
Wireline (or other communications conduit) operations present a special problem in “protecting” the wellbore from foreign objects. Typically a hole cover or other prophylactic device can be placed over the open hole, but when operations are proceeding, this is not an option. A device that prevents the inclusion of foreign objects into the wellbore while still allowing the reciprocating of communications conduit therein is needed.